In recent years the use of composite pressure vessel to manage hydrocarbons under high pressure has become more current. Such vessels should preferably be usable for a broad spectre of hydrocarbons, from dry, compressed natural gas, so-called CNG, to unprocessed well fluids from oil production. Well fluids would typically be an oil based liquid under pressure, containing compressed gas as well as pollution in the form of water, sand, H2S, CO2, etc. In order to make optimal use of the storing facility for the pressure vessel, dry gas is stored at low temperatures since this increases the compressibility of the gas, whilst the well fluids would typically have a high temperature from the reservoir. It is advantageous to maintain a high temperature for the well fluids in order to avoid problems with hydrate formation inside the pressure vessel and connecting pipe systems. In order to make optimal use of the storing volume onboard a vessel, it is advantageous to use large vessels, typically cylindrical vessels with a diameter greater than 2 meters and the height or length greater than 10 meters.
Composite pressure vessels comprises an inner liner produced from thermo plastic, such as high density polythene, HDPE or from hardened polymer materials, for example of the epoxy type. The inner liner acts as gas/fluid barrier in the vessel. Outside the inner liner, there is a composite layer maintaining the pressure inside the vessel, which is typically produced from filament of glass fibre or carbon fibre, wound with pretension on the vessels inner liner and embedded in a polymer material, for example of the epoxy type. At one end or both ends of the composite pressure vessel, there is an end boss, typically of metal and which is held in place by the composite structure. The metal end boss makes it simple to arrange feed-throughs for loading and unloading of the vessel and provides an entry into the vessel. For vessels with a broader use as mentioned above, the design and connecting of the end boss provide a significant challenge. The metal end boss has a significantly lower thermal expansion than polymer and composite materials, resulting in problems at prolonged use with large temperature and pressure variations. At high temperature the inner plastic liner will expand more than the metal end boss, resulting in tension build-up at the contact surface between the inner liner and the end boss. Due to the viscoelastic properties of polymer materials, prolonged use at high temperature will result in tension relief over time. When the pressure and temperature are later reduced, the inner liner will contract more than the end boss, resulting in that over time a gap is created in the contact interface between the inner polymer liner and end boss.
Patent publications WO 2005/093313, U.S. Pat. No. 5,938,209, WO 94/23240 and U.S. Pat. No. 5,287,988, describe different end boss constructions and sealing arrangements between the end boss and composite pressure vessel. Described are seal rings of elastic polymer material, constructions with an elastomer O-ring for sealing; and constructions where the inner polymer liner is moulded around and to an end boss having a rough surface. There is not a description of constructions where the inner liner is pressed against an inner metal counter pressure hold by means of pre-tensioning arrangement placed outside the inner polymer liner. Neither is there a description of a pre-tensioning arrangement, which may easily be adjusted should this be required after prolonged use.
There is a need for an end boss for a composite pressure vessel with more advantageous qualities than previously achievable; and a composite pressure vessel with such an end boss.